Add type safe cast to Option of singleton type. about shapeless HOT 7 CLOSED

@milessabin how did you envision this being implemented? I assumed the implementation would check for universal equality with the value of the Witness of the singleton type, but it doesn't look like that works. assertEquals(Witness(0).narrow, 0) results in expected: <shapeless.SingletonTypesTests$fresh$macro$3$1@60aff886> but was:<0>. Am I thinking about this wrong?

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@ceedubs Umm ... did you mean Witness(0).value?

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@milessabin ah yes that's what I meant. That makes my assertEquals example pass.

Unfortunately, I still can't get my implementation to work. It seems like it should be straightforward so I'm probably doing something silly. Maybe I'll let someone else take this one :)

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Do you have a branch or a gist we could take a look at?

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To be honest I don't have a firm understanding of singletons and witnesses; they still seem like magic to me. So I'm probably doing something quite foolish, but with risk of embarrassing myself, here it goes.

I had added this to SingletonOps:

def extract[V >: T](v: V): Option[T] = if (v == witness.value) Some(witness.value) else None

However, I haven't succeeded in getting this to return anything other than None. Also type refinements seem to be causing trouble when trying to check that the return type matches an Option of the singleton type.

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The following works for me,

scala> def cast(w: Witness)(x: Any): Option[w.T] = if(x == w.value) Some(w.value) else None
cast: (w: shapeless.Witness)(x: Any)Option[w.T]

scala> cast(0)(1: Any)
res0: Option[Int(0)] = None

scala> cast(0)(0: Any)
res1: Option[Int(0)] = Some(0)

I was thinking that we should have Typeable instances for types with an available implicit Witness, and then some additional typeable syntax that adds something like foo.cast(<literal>) where the literal singleton type is extracted from the inferred Witness as in cast above.

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Fixed in 7eb430b.

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